The present invention relates to an arrangement for mounting a protection diode on a vertical multijunction photovoltaic diode structure and a photovoltaic cell obtained by this process.
FIG. 1 shows schematically a conventional solar cell circuit. An assembly of photovoltaic diodes in series 1 feeds into a battery of accumulators 2 serving as a buffer battery for continuing to supply energy when the solar cells 1 are not illuminated. It is furthermore necessary to provide a protection diode 3 so that, when the photovoltaic diodes 1 are not illuminated, the battery 2 is not discharged by these diodes.
In practice, two large groups of solar cells are distinguished, the solar cells operating with normal light and solar cells operating with concentrated light. Among these latter, we will be here concerned more particularly with the laterally illuminated vertical multijunction cells, which are described for example in the article by B. L. Sater et al published in the work entitled "Solar Cells", edited by C. E. Backus, I.E.E.E. Press (1976).
There will be recalled rapidly with reference to FIGS. 2, 3 and 4 the structure of such vertical multijunction diodes. This structure is produced from silicon wafers 10 comprising a substrate 11 with a low doping level, covered on each of its faces with a layer of opposite conductivity type (respectively P and N 12 and 13).
As shown also in a side view in FIG. 3, a large number of wafers 10 is then stacked and these wafers are fixed to each other by means of a metal alloy. As the top view of FIG. 4 shows, this stack of wafers is then cut up for example by means of a saw along the lines designated by reference numerals 20 and 21. Thus an assembly of elementary vertical multijunction bars or cells is obtained which may be used with concentrated light by lateral illumination. Then, since the height of the stack, which may be relatively high and comprise more than 100 wafers, is nevertheless limited, it may be desired to mount in series or in parallel, an assembly of elementary cells such as those obtained subsequent to the operation described with reference to FIG. 4.
The elementary cell or the assembly of elementary cells mounted in series or parallel corresponds to assembly 1 shown in FIG. 1. It is then a matter of connecting to this assembly diode 3 which is conventionally effected by means of an external connection in the devices of the prior art. Furthermore, in the case where several elementary cells are mounted in parallel, if it is desired to avoid the risks of discharging one assembly of cells in another should one of these elementary assemblies not be suitably illuminated, it is advisable to dispose inversely connected diodes such as diode 3 in series with each elementary cell before mounting them in parallel, which considerably complicates the mounting operations.
Thus, an object of the present invention is to provide a process for the reverse mounting of a protection diode in a vertical multijunction photovoltaic cell structure which simplifies the mounting procedure and avoids the user making any possible error of mounting.